Neutrophil (PMN) migration within tissues is a central component of both host defense and many pathophysiologic processes. While many studies have focused on defining events involved in the regulation of PMN migration at the level of the cell surface, the sequence of events that serve to fine-tune PMN migration during the inflammatory response are incompletely understood. Perhaps even less well appreciated is how the pathways controlling PMN migration contribute to pathologic inflammation. Indeed, a variety of human diseases, in their active and symptomatic stages, are characterized by migration of large numbers of neutrophils (PMN) through tissues and across mucosal surfaces. Recent studies from our group investigating the mechanisms of PMN migration across mucosal surfaces demonstrated an important role of a membrane protein termed CD47 in regulating the rate of PMN migration. As outlined in this proposal, we have evidence demonstrating that CD47 binding to a cellular receptor termed signal regulatory protein (SIRPa) regulates the rate of PMN migration. Furthermore, we have data suggesting that several SIRPa proteins are present within different subcellular compartments in PMN. The overall goal of this proposal is define these PMN SIRP proteins and their role(s) in regulating PMN migration. To achieve this goal, we will identify region(s) on SIRPa that are involved in modulation of PMN migration, characterize SIRP proteins that are expressed in PMN and define their cellular distributions during chemoattractant stimulation. Information from the proposed studies will provide new insights into the role of SIRPs in the regulation of PMN migration and may yield new ideas for therapeutics aimed at inhibiting pathologic inflammation.